1. Field of the Invention
The present invention relates to digital data coding and decoding, and more particularly, to a digital data coding/decoding method and apparatus for coding/decoding digital data sliced into units of digits, in the order of the significance of the digits. The invention has been adopted in ISO/IEC JTC1/SC29/WG11 N1903 (ISO/IEC 14496-3 Subpart 4 Committee Draft).
2. Description of the Related Art
In general, a waveform including information is basically a continuous analog signal. To express the waveform as a discrete signal, analog-to-digital (A/D) conversion is necessary.
To perform the A/D conversion, two procedures are necessary: 1) a sampling procedure for converting a temporally continuous signal into a discrete signal; and 2) quantizing procedure for limiting the number of possible amplitudes to a limited value, that is to say, for converting input amplitude x(n) into an element y(n) belonging to a finite set of possible amplitudes at time n.
Since the quantized signal is usually simply coded by a PCM (Pulse Code Modulation) method, which does not require another process, it is very convenient to use the quantized data. However, in view of the magnitude of data typically necessary for storage or transmission, this simple coding method is not optimal even if input samples are statistically independent. Further, if input samples are statistically independent from one another, the sample coding method is more inappropriate. Thus, coding is performed, including lossless coding such as entropy coding or a certain kind of adaptive quantization. Therefore, the coding procedure becomes very complex, compared to the simple PCM data storing method.
A bitstream includes quantized data and side information for compressing signals. However, such information forms bitstreams by coding the quantized data in the unit of samples in the order of data, without considering the significance of the bits. If the bitstreams are simply stored and then restored without any error, the bitstreams may be generated irrespective of the significance of the bits.
When the bitstreams are transmitted via a communications network, parts of the bitstreams may be lost according to the state of the communications network. Also, if an error is generated during transmission of the bitstreams, the information of the bitstreams after the error is generated is restored as erroneous information, due to propagated error. If only parts of the bitstreams among all transmitted bitstreams are restored as correct information, and more significant signals are restored in the bitstreams restored as correct information compared to unrestored bitstreams, deterioration in quality is to a lesser degree.
According to general coding techniques, a fixed bitrate is given to a coding apparatus, the optimal state for the given bitrate is searched to then perform quantization and coding, thereby forming bitstreams in accordance with the bitrate. In a conventional coding technique, bitstreams having a magnitude suitable for a given bitrate are formed, without considering the order of bitstreams.
Actually, if the thus-formed bitstreams are transmitted via a communications network, the bitstreams are sliced into several slots to then be transmitted. When an overload is applied to a transmission channel, or only parts of slots, not all slots, sent from a transmission end, are received at a reception end due to a restricted bandwidth of the transmission channel, data cannot be reconstructed properly. Also, if only some of the bitstreams are restored, the quality is severely degraded. In the case of digital audio data, sound objectionable to the ear is reproduced. In the case of digital video data, an image reconstructed on the screen is severely impaired.